first of all, the exam is finished, if you have more exams, worry about them. if not, enjoy post-hsc life.
only one question, were your school's ranks close at the top?
if so, there's a good chance that you can get pulled up a couple marks, because someone from your school might have had a better day. that can pull your internal mark considering your ranked first. Your estimate for your exam mark shouldn't be too far off the b6 cutoff.
So stay optimistic and hope that everything works out